The present invention relates to thermal dye-transfer receiving elements having at least one oriented layer comprising a continuous phase or matrix of a first polymer having microbeads of a second polymer dispersed therein which microbeads are at least partially bordered by voids. In particular, the second polymer is selected to provide microbeads that have a Tg below 80xc2x0 C. The related method of making a thermal dye transfer element is also disclosed.
In recent years, thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-transfer receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are set forth in U.S. Pat. No. 4,621,271.
Thermal dye-transfer receiving elements used in thermal dye transfer generally comprise a polymeric dye image-receiving layer coated on a support. Supports are required to have, among other properties, adequate strength, dimensional stability, and heat resistance. For reflective viewing, supports are also desired to be as white as possible. Cellulose paper and plastic films have been proposed for use in the supports for the dye-transfer receiving element in efforts to meet these requirements. Recently, microvoided films formed by stretching an orientable polymer containing an incompatible organic or inorganic material have been suggested for use in dye-transfer receiving elements.
Various arrangements have been proposed to improve the imaging quality of dye-image receiving layers in thermal dye-transfer receiving elements. For example, JP 88-198,645 suggests the use, in a dye donor element, of a support comprising a polyester matrix with polypropylene particles as a microvoiding agent. EP 582,750 suggests the use of a non-voided polyester layer on a support.
U.S. Pat. No. 5,100,862 relates to microvoided supports for dye-receiving elements used in thermal dye-transfer systems. Polymeric microbeads are used as void initiators in a polymeric matrix to enable higher dye transfer efficiency. A problem exists with such support, however, in that in order to attain the high level of voiding necessary for desired dye transfer efficiency, the volumetric loading of the microbeads needs to be above 25% by volume of the polymeric matrix. The degree of voiding is preferably from about 30 to 60 volume percent. At these levels of loading, the tear strength of the film during manufacture is very low and results in very poor manufacturing efficiency due to tearing of the support.
U.S. Pat. No. 6,096,684 relates to porous polyester films suitable as supports for receiving elements used in thermal dye transfer systems. Polymers immiscible with a polyester are used in a base layer while an adjacent layer, upon which a dye-image receiving layer is formed, contains a polyester containing dispersed inorganic particles as void initiators. These inorganic particles are less than 1.0 xcexcm in size. The porosity of this adjacent layer is specified to be not less than 20% by volume. This support solves the problem of poor adhesion of imaging layers to a support consisting only of the base layer. This support has also been shown to be manufacturable at high efficiency. A problem exists with this support, however, in that the hardness of the inorganic void initiators results in poor contact with the dye donor element. This results in low dye transfer efficiency for elements using such supports.
Blends of linear polyesters with other incompatible materials of organic or inorganic nature to form microvoided structures are well-known in the art. U.S. Pat. No. 3,154,461 discloses, for example, linear polyesters blended with, for example, calcium carbonate. U.S. Pat. No. 3,944,699 discloses blends of linear polyesters with 3 to 27% of organic material such as ethylene or propylene polymer. U.S. Pat. No. 3,640,944 also discloses the use of poly(ethylene terephthalate) blended with 8% organic material such as polysulfone or poly(4-methyl-1-pentene). U.S. Pat. No. 4,377,616 discloses a blend of polypropylene to serve as the matrix with a small percentage of another incompatible organic material, nylon, to initiate microvoiding in the polypropylene matrix. U.K. Patent Specification 1,563,591 discloses linear polyester polymers for making opaque thermoplastic film support in which has been blended finely divided particles of barium sulfate together with a void-promoting polyolefin, such as polyethylene, polypropylene or poly-4-methyl-1-pentene.
The above-mentioned patents show that it is well known to use incompatible blends to form microvoided films after such blends have been extruded into films and the films have been quenched, biaxially oriented and heat set. The minor component of the blend, due to its incompatibility with the major component of the blend, upon melt extrusion into film, forms generally spherical particles each of which initiates a microvoid in the resulting matrix formed by the major component. The melting points of the void initiating particles, in the use of organic materials, typically should be above the glass transition temperature of the major component of the blend and, in particular, above the temperature of biaxial orientation.
As indicated in U.S. Pat. No. 4,377,616 spherical particles initiate voids of unusual regularity and orientation in a stratified relationship throughout a matrix material after biaxial orientation of the extruded film. Each void tends to be of like shape, not necessarily of like size, since the size depends upon the size of the particle. The voids generally tend to be closed cells, and thus there is virtually no path open from one side of a biaxially oriented film to the other side through which liquid or gas can traverse. The term xe2x80x9cvoidxe2x80x9d is used herein to mean devoid of solid matter, although it is likely the xe2x80x9cvoidsxe2x80x9d contain a gas.
Upon biaxial orientation of the resulting extruded film, the film becomes white and opaque, the opacity resulting from light being scattered from the walls of the microvoids. The transmission of light through the film becomes lessened with increased number and with increased size of the microvoids relative to the size of a particle within each microvoid.
U.S. Pat. No. 3,944,699 also indicates that the extrusion, quenching and stretching of the film, in this case made from a polyester material, may be effected by any process which is known in the art for producing oriented film, such as by a flat film process or a bubble or tubular process. The flat film process involves extruding the blend through a slit dye and rapidly quenching the extruded web upon a chilled casting drum so that the polyester component of the film is quenched into the amorphous state. The quenched film is then biaxially oriented by stretching in mutually perpendicular directions at a temperature above the glass transition temperature of the polyester. The film may be stretched in one direction and then in a second direction or may be simultaneously stretched in both directions. After the film has been stretched, it is heat set by heating to a temperature sufficient to crystallize the polyester while restraining the film against retraction in both directions of stretching.
It has previously been taught that microbeads should have a glass transition temperature Tg of at least 20xc2x0 C. higher than the Tg of the continuous phase polymer matrix. See U.S. Pat. Nos. 4,994,312; 5,143,765; and 5,156,905 and Re. 34,742.
Commonly assigned, copending U.S. Ser. No. 10/033,457, filed Dec. 27, 2001, is directed to shaped articles comprising an oriented first polymer continuous phase having dispersed therein crosslinked microbeads which are at least partially bordered by void space, wherein the monomers from which the microbeads are derived are selected to provide microbeads that are both low-yellowing and thermally stable.
It would be desirable to solve the problem, existing in the prior art, of surface irregularities, in thermal dye-transfer receiving elements, that cause image quality problems. It would be desirable to have a receiving element for thermal dye transfer that can be readily manufacturable and exhibits a high dye transfer efficiency
The invention provides thermal dye-transfer receiving elements (xe2x80x9creceiversxe2x80x9d) comprising at least one microvoided layer that employs, as the voiding agent, crosslinked polymer microbeads, wherein the glass transition temperature of the microbeads is below 80xc2x0 C. Applicants have found that more compliant microbeads can improve the image properties of the thermal dye-transfer element. In particular, improved dye efficiency and improved low-density uniformity can be obtained using the present invention.
In one preferred embodiment, the Tg of the microbeads is low enough to soften during the thermal printing process. The invention also provides a method of making thermal dye-transfer receivers.